A suspension system component or the like of an automobile is produced by performing press processing or other processing on a metal sheet such as a high-tensile steel sheet. In the production process, burring processing that forms a burring hole in the metal sheet may be performed. Conventional burring processing is performed by first forming a punched hole 52 in a metal sheet 51 as shown in FIG. 1, and then forming a vertical wall 53 by expanding the punched hole 52 by force as shown in FIG. 2. In the press process of the vertical wall 53 (hereinafter, “burring process”), large tensile stress acts on the punched hole 52 at the time of expanding the punched hole 52 by force. Hence, in the conventional burring processing method, there has been a case where cracking (hereinafter, “burring cracking”) occurs in the vertical wall 53 in the burring process.
To solve this problem, Patent Literature 1 discloses a method in which drawing processing is performed in a press process of the first time so that the metal sheet has a U-shaped cross-sectional shape, then a punched hole is formed in the bottom surface in a press process of the second time, and a vertical wall is formed in a press process of the third time.
Further, Patent Literature 2 discloses a method in which drawing processing is performed while a drawing shoulder radius of a press process of the first time is set large, then the drawing shoulder portion is re-struck in a press process of the second time, and the entire bottom surface is punching-processed in the last stage of the processing.